Since a study on conductive polymers was reported for the first time in 1977 (H. Shirakawa et al., Chem. Commun. 578, 1977), research on identifying the conductive or semi-conductive features of organic materials and polymers, in addition to their previously verified insulator properties, has been actively carried out.
As shown in FIG. 1, organic thin film transistors, one of such organic electronic device fields, function as the driving circuit of an organic device, such as a display, based on the principle that, when a voltage is applied to a gate electrode, electric charges are accumulated due to a local polarization effect caused by the dielectric properties of a gate insulator layer, and the thus accumulated electric charges forms a channel between the gate insulator layer and an organic activation layer. While when a voltage is applied to the region between the source and the drain, a driving state (“on” state) where the electric charges flow is made, when a reverse voltage (0 voltage) is applied thereto, a non-driving state (“off” state) where the electric charges do not flow is made even though a voltage is applied to the region between them.
Further, since organic thin film transistors utilize low molecular organic materials or polymers as a structural component unlike conventional silicon-based transistors, they have the advantage that they can be applied to various fields of flexible organic devices such as flexible displays of the next generation.
In such an organic thin film transistor, the dielectric properties of a gate insulator layer play an important role in the device performance. By controlling such dielectric properties, the local polarization effect is maximized, enabling an increase in the accumulation of dielectric charges under low voltage conditions, resulting in low voltage operation. Further, when a gate insulator layer with high permittivity is used, the thickness of a gate insulator layer can be increased without the lowering of capacitance, and a high output current can be stably maintained while preventing the generation of a leakage current, which is a chronic problem of organic devices, thereby making it favorable for the commercialization of organic thin film transistor as a driving semiconductor.
However, as compared with the conventional silicon-based transistors, organic thin film transistors show relatively low charge carrier mobility and have a low on/off current ratio where their current at the on state is low, while their current at the off state is high. This is because it is difficult to fabricate a gate insulator layer with a high permittivity by using organic polymers. Consequently, high dielectric inorganic materials are used as the gate insulator, where the biggest advantage of organic thin film transistors, i.e., flexibility, must be sacrificed.